The present invention relates to subterranean drilling operations, and more particularly, the present invention relates to methods and compositions for reducing the permeability of a subterranean formation to aqueous-based fluids during the drilling phase.
The process of drilling a well bore in a subterranean formation typically requires the use of a drilling fluid. During the drilling process, the drilling fluid is passed down through the inside of the drill string, exits through the drill bit, and returns to the surface through the annulus between the drill string and the well bore. Among other things, the circulating drilling fluid lubricates the drill bit, carries drill cuttings to the surface, and balances the formation pressure exerted on the well bore. This process in the life of a well is known as the “drilling phase.” It is understood that there is also a “production phase” in the life of a well, during which hydrocarbons or other desired substances may be produced.
While well bores usually are drilled in hydrocarbon-producing formations, the formations may contain layers of water or may be located adjacent to water-producing zones. The high mobility of water may allow it to flow into the well bore by way of natural fractures and/or high permeability streaks present in the formation. Over the life of such wells, the ratio of water to hydrocarbons recovered often becomes so high that the cost of producing the water, separating it from the hydrocarbons, and disposing of this water may represent a significant economic loss. Besides being highly undesirable during the production phase, water-producing zones may cause problems in the well bore with certain completions activities. However, these problems with the production of undesired water generally are not addressed during the drilling phase.
To reduce the production of undesired water from hydrocarbon-producing formations, aqueous polymer solutions that may contain cross-linking agents have been used. Such polymer solutions are injected into the formation and cross-linked to form stiff gels that may stop or reduce the flow of the undesired water. Even when a polymer solution is properly placed in a water-producing section, however, the cross-linked gels formed may not remain stable in the zone due to thermal degradation and/or differences in the adsorption characteristics of the polymer, associated cross-linker, and the like. Further, the selected placement of a polymer solution in a producing formation may require expensive, time-consuming mechanical zonal isolation. Zonal isolation also may be inaccurate, which may lead to plugging and/or damaging the hydrocarbon-bearing sections. Damage to hydrocarbon producing pathways is undesirable.
Recently, wells in the production phase have been treated with compounds to reduce the production of water with hydrocarbons. These compounds are commonly referred to as “relative permeability modifiers.” Relative permeability modifiers, such as polyacrylamide, may be dissolved in water and pumped into a subterranean formation that produces water and hydrocarbons, reducing the permeability of the formation to water without substantially affecting the permeability therein to hydrocarbons. The use of these relative permeability modifiers, however, has resulted in only small temporary reductions in water production and/or unacceptable levels of reduction in hydrocarbon production. Further, conventional relative permeability modifiers heretofore have not been used in the drilling phase.